1. Field of the Invention
This invention relates to industrial process control systems. Such systems comprise final operators such as valves, control devices such as electronic controllers, and process monitors such as temperature probes. Such systems are used to adjust the value of a first parameter, such as flow rate into a boiler, in response to the value of a second parameter such as temperature within the boiler.
The final operator varies the value of the first parameter in response to a control signal which is generated by the control device in response to a measurement signal generated by the process monitor. The control signal value is related to the difference between the measurement signal and a set point signal, which represents the actual and desired operating points of the process respectively.
The value of the set point is selected by either a process engineer or computer to optimize the process, and is based upon calculations not relevant to the disclosure. It is necessary to display the set point and measurement values for use by the process engineer as an indication of the degree to which the actual operating point of the process conforms to the desired. In the case of a computer generated set point, the availability of this information allows the engineer to take over control of the process in case of computer failure.
This invention describes devices used to display these values. The quantity displayed may be the actual parameter value, the value of the control system signal representing the parameter, or a dimensionless quantity related to the signal or parameter ranges known as "percent span".
In this discussion the term "percent span" or simply "percent" will be used for the sake of clarity. The actual range of value the signal is allowed to assume is merely a characteristic of the particular control system chosen and may, for example be 0-10 volts, 4-20 milliamps, or 3-15 psi. The parameter range represented by a signal range is a characteristic of the particular process and dependant on not only the actual parameter, such as flow rate or temperature, but on the expected parameter variation. Thus, one process may require a parameter range of temperature of 25-50.degree.F or -20 - 100.degree.F.
To avoid the confusion resulting from many combinations of signal and parameter ranges possible, it is conventionally acceptable to represent and discuss these values in terms of their span. Span is the algebraic difference between the minimum and maximum value of the parameter or signal. For example, a temperature range of -20 to 100.degree.F has a span of 120.degree., while a signal range of 0 to 10 volts has a span of 10 volts. For a control system in which a 0-10v range represents -20 to 100.degree.F, 25% span corresponds to 10.degree.F, if referring to parameter value, and to 2.5v if referred to signal value. For the purpose of this discussion, both values would be referred to as a parameter value of 25% span or a signal value of 25% span. The indicators referred to thereby display a range of 0-100% span, although it is understood that it is obvious to display only a portion of the span.
This invention relates to electronic display devices used to indicate measurement and set point values. Indicators for set point and measurement displays are generally of two types. The first type presents a digital readout in response to a signal imposed on logic circuitry, and includes CRT's, light emitting diodes, and Nixie .sup.(TM) tubes.
The second type of display utilizes a meter type indicator which provides a voltmeter or ammeter for positioning a pointer on a scale in response to the voltage or current value of a monitored signal. The actual signal monitored is dependant upon the type of meter used and represents the parameter to be controlled.
One type of meter used to indicate set point or measurement value is an absolute value meter, which monitors the set point or measurement signal and positions the pointer in response to the signal magnitude. The scale range is, of course, 0 to 100% span, with the 0% and 100% points at the opposite endpoints of the scale. Although absolute meters are inherently accurate at the 0% span point, and may be calibrated for accuracy at a second point, the accuracy at all other points is limited by errors resulting from the non-linearity which is characteristic of such meters. These problems are costly to overcome, and generally limit the usefullness of absolute meters where accurate displays are required. For example, an absolute meter having a typical accuracy of .+-.5% may indicate 25% span in response to an actual measurement value of 20-30% span.
To overcome the problem, deviation meters were used to indicate the algebraic difference between the set point and the measurement values, hereinafter called the deviation value. Deviation meters monitor the deviation signal and position a pointer in response to both the magnitude and polarity of the deviation value. The scale range is minus 10% span to plus 10% span with a centrally located 0% span point. If, for example, the set point value is 50% span and the measurement value is 60% span, the deviation meter will indicate +10% span on its scale. A change in the measurement signal to 40% span will result in an indication of -10% span on the scale.
Errors due to non-linearity are greatly minimized by use of a deviation meter coupled with a particular characteristic associated with process control systems. Under normal conditions, the set point and the measurement values of a control system will be approximately equal, and the deviation signal will normally be just a few percent of span. Thus, a deviation meter providing an indication of .+-.10% span and having the typical accuracy of .+-.5%, would display a deviation value accurate to .+-.0.5% span. If the set-point value is known, the measurement value may be ascertained by simply adding the indicated deviation value to the value of the set-point.
This invention specifically relates to display devices using deviation meters to indicate measurement and set-point values.
2. Description of the Prior Art
The use of deviation meters in set-point and measurement displays is known in the art. Typically, the deviation meter positions a pointer on a deviation scale to indicate the polarity and magnitude of the monitored deviation signal. The measurement value may then be determined by adding the deviation value to the set-point value. That addition may be performed mentally, or the display configuration may correlate the deviation scale to a set point scale in a manner which allows the measurement value to be determined by reference to the two scales.
One such configuration known in the art has a front panel containing a linear deviation scale and a proximately located set-point adjustment knob. The linear scale has a centrally located null point of 0%, and two end points of +10% span and -10% span. Associated with the linear scale is a fixed pointer, located at the null point, and a movable pointer which is positioned by a deviation meter and moved from the null point towards one of the endpoints in response to the magnitude and polarity of a deviation signal.
The set point adjustment knob may be rotated to adjust the set-point value, which is determined using the fixed pointer as a reference for a circular set point scale located on the dial surface.
In operation, the deviation and set point values may be determined from the linear and circular scales, respectively, and added together to obtain measurement values. The mental addition may be eliminated in a manner to be explained below, utilizing a series of parallel lines, etched in the front panel, which correlate points on the linear scale to points on the periphery of the circular scale. In operation, the set point dial is rotated until the desired set point value on the dial scale, appears under the etched line leading to the fixed pointer. The deviation scale positions the movable pointer in response to the magnitude and polarity of the deviation signal, to indicate the deviation value which may then be converted to the actual measurement value by following the particular etched line from linear scale position to the periphery of the dial. The dial scale value at the end of the line is the actual measurement value.
Besides being rather cumbersome, the above configuration has a limitation in that the lines extending from the linear scale may only correlate that portion of the dial scale within +90.degree. of the set point value. Further, the increments between dial scale units, as transposed to the linear scale, become smaller as distance from the null position increases, resulting in difficult interpolation. In practice, only measurement values within .+-.30% span of set point could be read in conjunction with three inch linear scale.
A second configuration known in the art partially overcomes the difficulties in readability associated with the previously described configuration by using a deviation meter to display the actual measurement value. This second configuration uses a tape having a set point scale printed thereon and movable between two reels in a manner similar to film in a movie projecter or tape in a tape recorder.
A portion of the tape may be viewed through a window located between the two reels, the window including a fixed indicating pointer. Adjustment of the set point value rotates the reels, causing the proper set point value to appear under the indicating pointer, and exposing the surrounding portion of the tape from which measurement signals may be read. A movable pointer, having a null position co-incident with the position of the fixed pointer, is moved from its null position in response to the magnitude and polarity of a deviation signal. The movable pointer is positioned on the exposed portion of the tape at the scale point corresponding to the measurement value, said point being a distance from the set point scale point which is proportional to the deviation signal. This configuration provides an effectively large scale within a relatively small area, but suffers from several disadvantages. As the effective length of the scale increases, the portion of the tape exposed through the window represents a less permissable variation of the measurement signal around the set point value. Further, the configuration necessitates a complex construction.